Polyacrylamide (PAM) and copolymers thereof with acrylic acid (PAMAA) are well known in the industry for a plethora of applications. Commercially important applications of these copolymers include their use as flocculants in water treatment or papermaking processes, as rheological additives for water or waterbased solutions in applications such as enhanced oil recovery (EOR), as water absorptive agents when dried, and as friction reduction agents in waterbased formulations.
In dilute aqueous solutions, such as 1 wt % or less commonly employed in EOR applications, PAM and its copolymers are susceptible to chemical, thermal, and mechanical degradation. The conditions encountered in EOR applications can include one or more of high shear, harsh chemical environments, and temperatures of 70° C. and higher. Chemical degradation occurs when the amide moiety hydrolyzes at elevated temperature and/or pH, resulting in the evolution of ammonia and a residual carboxyl group. Thermal degradation of the vinyl backbone may occur through any one or more of several possible radical mechanisms. Mechanical degradation can also be an issue at the high shear rates experienced in the near-wellbore region, and within pumps and mixing devices designed to prepare the EOR solutions for injection into a reservoir.
Cross-linked variants of polyacrylamide have shown greater resistance to all of these methods of degradation, and have proved to provide viscosity stability in EOR applications. One commercially important type of crosslinked polymer used in EOR applications is PAMAA ionically crosslinked via interaction of the acrylic acid moieties with multivalent cations. Salts of Ca2+, Mg2+, Zn2+, Cr2+, and Al3+, for example, are employed commercially to form ionic crosslinks with the copolymers. In aqueous solutions, such polymers have increased viscosity compared to uncrosslinked polymers. Additionally, the crosslinks are capable of reforming after thermal or mechanical disruption during use. In such ionically crosslinked systems, it is desirable to employ a polymer having acrylic acid moieties arranged randomly throughout the copolymer, because this leads to maximum crosslink efficiency and the highest possible effective molecular weight of the crosslinked composition. As a practical matter, random acrylic acid placement in a copolymer leads to the observation that subsequent ionic crosslinking is efficient in raising the viscosity of aqueous solutions of the polymer; and a blockier copolymer requires more crosslinker to reach the same solution viscosity as a similar but more random copolymer.
In theory there are two ways to form poly(acrylamide-co-acrylic acid): by directly copolymerizing acrylamide and acrylic acid, or by post-treating an acrylamide polymer by partial hydrolysis to form carboxylate moieties; such polymers are often referred to in the industry as “HPAM”. Direct copolymerization of acrylamide and acrylic acid (or the conjugate base thereof) leads to blocky copolymers due to the large reactivity ratio differences. Rintoul and Wandrey, Polymer 46 (2005), 4525-4532 have reported polymerization reactivity ratios for acrylamide and acrylic acid as a function of several different variables. Reproduced below is a table showing the pH dependence of reactivity ratios r1 (acrylamide) and r2 (acrylic acid) in copolymerization reactions carried out at a total monomer concentration of 0.4 mol/L in water at 40° C.
Reactivity ratios of AM (r1) and AA (r2) at different pHs. Reaction conditions: T=313 K, [AM]+[AA]=0.4 mol/1, [K2S2O8]=1.8×10−2 mol/l. Source: Rintoul and Wandrey, Polymer 46 (2005), 4525-4532.
Reactivity ratiospHr1r21.80.541.482.70.691.343.60.821.284.41.270.915.31.830.516.22.500.397.82.950.428.83.050.42123.040.32
Commercially, preparation of PAMAA is often carried out by the post-treatment route, starting with PAM homopolymer. Hydrolysis may be carried out by synthesizing or dispersing a PAM homopolymer in water, adding a concentrated sodium hydroxide solution, and heating the mixture. However, as noted above hydrolysis of acrylamide functionality leads to evolution of ammonia (NH3), so special equipment is required to carry out the procedure. Additionally, the process requires a significant amount of heat energy.
Water-in-oil (w/o) latices are commercially significant sources of PAM and PAM copolymers for EOR applications due to their high solids content (up to 60-80 wt % polymer) combined with low viscosity and rapid inversion to use concentration, resulting in ease of use in the field. Due to the difficulty of post-emulsification, w/o latices of PAM or PAM copolymers are formed commercially by emulsifying the monomers and polymerizing in situ. Post-hydrolysis of the w/o latices requires formulation to impart latex stability during the hydrolysis step.
The post-treatment technique has the advantage of providing more randomly distributed carboxyl groups along the polymer backbone than is possible by conventional copolymerization of the two monomers as starting materials. See, e.g. M. Higuchi and R. Senju, “Kinetic aspects of alkaline hydrolysis of poly(acrylamide)”, Polymer Journal, Vol 3, No. 3, pages 370-377 (1972). When ionically crosslinked as described above, such post-hydrolyzed copolymers provide a higher viscosity at a given level of crosslinker than copolymers obtained by copolymerization of acrylamide and acrylic acid at neutral pH, in theory due to the increased randomness of the acrylic acid distribution when compared to copolymerization.
There is a need in the industry to provide PAMAA copolymers using methods that do not result in evolution of NH3. There is a need in the industry to provide PAMAA copolymers having increased solution viscosity when ionically crosslinked. There is a need in the industry to provide PAMAA latices, solutions, and dry powdered PAMAA polymers that are easily formed using conventional equipment. There is a need in the industry to provide ionically crosslinked PAMAA wherein the amount of crosslinker required to reach a target viscosity is less than the amount of crosslinker required to reach the same viscosity for a PAMAA copolymer formed via post-treatment hydrolysis of PAM.